Accurate measurement of heteronuclear dipolar couplings by phase-alternating R-symmetry (PARS) sequences in magic angle spinning NMR spectroscopy

We report a Phase-Alternating R-Symmetry (PARS) dipolar recoupling scheme for accurate measurement of heteronuclear H-1-X (X = C-13, N-15, P-31, etc.) dipolar couplings in MAS NMR experiments. It is an improvement of conventional C-and R-symmetry type DIPSHIFT experiments where, in addition to the dipolar interaction, the H-1 CSA interaction persists and thereby introduces considerable errors in the dipolar measurements. In PARS, phase-shifted RN symmetry pulse blocks applied on the H-1 spins combined with pi pulses applied on the X spins at the end of each RN block efficiently suppress the effect from H-1 chemical shift anisotropy, while keeping the H-1-X dipolar couplings intact. Another advantage over conventional DIPSHIFT experiments, which require the signal to be detected in the form of a reduced-intensity Hahn echo, is that the series of pi pulses refocuses the X chemical shift and avoids the necessity of echo formation. PARS permits determination of accurate dipolar couplings in a single experiment; it is suitable for a wide range of MAS conditions including both slow and fast MAS frequencies; and it assures dipolar truncation from the remote protons. The performance of PARS is tested on two model systems, [N-15]-N-acetyl-valine and [U-C-13, N-15]-N-formyl-Met-Leu-Phe tripeptide. The application of PARS for site-resolved measurement of accurate H-1-N-15 dipolar couplings in the context of 3D experiments is presented on U-C-13, N-15-enriched dynein light chain protein LC8. (C) 2014 AIP Publishing LLC.